The following year, he moved to Denver, Colorado, where he had gotten his first position as a lawyer.

Costigan was a founding member of the Progressive Party in Colorado in 1912, but lost the election for governor. President Woodrow Wilson appointed Costigan as a member of the United States Tariff Commission in 1917.

Colorado Democratic Senator Edward P. Costigan and New York Democratic Senator Robert F. Wagner sponsored a federal anti-lynching law in 1934. In 1935 Senate leaders tried to persuade President Franklin D. Roosevelt to support the Costigan-Wagner Bill. Roosevelt was concerned about a provision of the bill that called for the punishment of sheriffs who failed to protect their prisoners from lynch mobs. He believed that he would lose the support of the white voters in the South by approving this, and lose the 1936 presidential election.

The Costigan-Wagner Bill received support from many members of Congress but the Southern bloc managed to defeat it in the Senate. The national debate that took place over the issue again brought renewed attention to the crime of lynching. By the mid-1930s, the rate of the crime had finally dropped, mostly below 20 annually.